Titania, i.e. titanium oxide (TiO2), exhibits photocatalytic activity when subjected to light or the like applied thereto, and has been used as a material of coating films that are formed on the surfaces of various base materials such as ceramics, potteries, metals, fibers, resins, and building materials for the purpose of protecting the surfaces of the base materials and preventing, by virtue of its photocatalytic activity, the surfaces of the base materials from becoming dirty. As a technique of forming the titania coating film on the surface of the base material, there has been used, for example, a method of applying a solution containing non-crystalline titania onto the surface of the base material, drying it, and then calcining it, or a method of applying a solution containing crystalline titania onto the surface of the base material, and then heating to dry it at a low temperature.
As the foregoing solution containing non-crystalline titania, there is known, for example, such a solution that is obtained by dispersing fine particles of amorphous titania having a property of no existence of particle boundary into aqueous hydrogen peroxide. On the other hand, as the solution containing crystalline titania, there is known, for example, such a solution that is obtained by dispersing fine particles of anatase titania into water. Examples of conventional methods of producing these solutions will be briefly described hereinbelow.
First, description will be given about the method of producing the solution containing amorphous titania, i.e. non-crystalline titania. Titania is produced by mixing titanium tetraisopropoxide (TIP) and isopropanol (IPA) together at a predetermined mole ratio and agitating the mixture, then adding a predetermined amount of a liquid mixture of IPA and water into the mixture and agitating it. Then, the produced titania is separated from the mother liquid and then is subjected to airing to be dried, thereby obtaining powder of titania. Thereafter, the obtained powder is dissolved in aqueous hydrogen peroxide to produce a titania gel (a gel of titania), then aqueous hydrogen peroxide is further added as a dispersing agent to transform the titania gel to a titania sol (a sol of titania), thereby obtaining the titania solution containing fine particles of amorphous titania (e.g. see Patent Literature 1).
The solution containing amorphous titania, thus obtained, is applied in the form of a thin film onto the surface of a base material such as ceramics or metal and dried, then calcined at a predetermined temperature such as at 500° C. to transform amorphous titania to anatase titania, thereby forming a titania coating film that exhibits photocatalytic activity, on the surface of the base material (e.g. see Patent Literature 2).
On the other hand, the solution containing anatase titania, i.e. crystalline titania, is produced using, for example, a solution in which titanium hydroxide is dispersed, as a material. At the outset, ammonia and sodium hydroxide are added into an aqueous solution of inorganic titanium compound such as titanium chloride or titanium sulfate to produce a titanium hydroxide gel. After fully washing the produced titanium hydroxide gel, aqueous hydrogen peroxide is added to thereby obtain a titanic acid solution. Non-reacted hydrogen peroxide is decomposed on standing. Then, by applying a heat treatment to the obtained solution at a predetermined temperature for a predetermined time, crystalline nuclei of anatase are produced to thereby obtain the solution containing anatase titania. The solution containing anatase titania, thus obtained, is not required to be subjected to calcination as opposed to amorphous titania inasmuch as the contained anatase titania already has a property of exhibiting photocatalytic activity. Therefore, by applying the solution containing anatase titania onto the surface of a base material, and then drying it or heating it at a low temperature, a titania coating film that exhibits photocatalytic activity is formed (e.g. see Patent Literature 3).
[Patent Literature 1] JP-A-H10-167728
[Patent Literature 2] JP-A-H10-46317
[Patent Literature 3] JP-A-H10-67516
The foregoing solutions containing titania have, however, the following problems. Specifically, if, for example, the solution containing amorphous titania or anatase titania is left to stand for a long time, titania may be aggregated in the solution so that a high dispersion state of titania can not be maintained. For example, in case of the solution containing amorphous titania, since amorphous titania is dispersed in aqueous hydrogen peroxide as described above, the solution has a strong acidity with a pH of, for example, about 1. With such a strong acidity, amorphous titania may be subjected to condensation polymerization so as to be gelled or aggregated. Specifically, if, for example, the solution is left to stand at normal temperature, the viscosity of the solution may become high in several days, and further, titania may be gelled or lumped. On the other hand, in case of the solution containing anatase titania, particles of titania may be mutually attracted to be aggregated. Further, since titania is dispersed in aqueous hydrogen peroxide in the preparation process of the titania solution, if the heat treatment for crystallization is carried out in the state where hydrogen peroxide remains, titania before crystallization may be gelled or aggregated particularly in the initial stage of the heat treatment.
As another problem, in preparing either of the amorphous and anatase titania solutions, when titania is dissolved in aqueous hydrogen peroxide, the temperature of the solution rises due to heat of the dissolution so that hydrogen peroxide may be actively decomposed to produce, for example, bubbles of oxygen gas. Since these bubbles may become the cause of, for example, roughening the coating film, it is desirable to remove them. However, inasmuch as it takes time and labor for such removal, the production efficiency may be lowered. Further, if the solution is prepared in large quantity, the calorific value becomes large to boil the solution, which is dangerous for an operator.
As another problem of the solution containing amorphous titania, since, as described above, the solution shows the acidity with the pH of, for example, about 1, selection of base materials applicable with the coating is limited. Particularly, if the metal base material is selected and applied with the titania solution, the surface thereof may be corroded. Further, since amorphous titania itself can not exhibit photocatalytic activity, when amorphous titania is used as the material of the coating film, calcination is required after the application of the titania solution onto the surface of the base material to thereby transform amorphous titania to anatase titania in order to obtain photocatalytic activity. Consequently, there are those instances where sufficient calcination can not be carried out with respect to a base material having a low heat resistance, or a large base material such as a building. Moreover, in order to produce anatase titania by heating the solution containing amorphous titania as described in Patent Literature 3, the heat treatment is required for a long time so that it is difficult to improve the productivity.